1. Field of the Invention
This invention relates to cementing of a casing string in a wellbore, and more particularly to a fluid loss additive for addition to a cement slurry to be used in the cementing job. It is common practice in the art of oil and gas production to cement a casing string or liner in a wellbore by pumping a cement slurry down the wellbore and into the annulus between the casing string and the exposed borehole wall. If the cement comes into contact with permeable formations, and a pressure differential exists toward the formation, then the water in the cement can be forced into the permeable formation. The cement particles are so large that they cannot be pumped into the matrix of the rock, so the solids remain in the annulus. When this happens, the water-to-solids ratio in the cement slurry is reduced, and the cement slurry starts to thicken, with simultaneous cement volume reduction. This unwanted fluid loss from the cement can cause a variety of problems, one of the worst being that the last of the cement cannot be displaced out into the annulus due to the high viscosity of the reduced-water portion of the cement. Because of the magnitude of problems which can be encountered with poor fluid loss control, fluid loss control additives are commonly used in oil well cementing.
The ideal fluid loss additive would be of low cost and have no adverse effects on the performance properties of cement. For a cement slurry to seal the annulus, it must be effectively "placed," then changed from a liquid to a solid in the annulus.
Most fluid loss additives are water-soluble polymers. These polymers work in conjunction with the cement particles themselves to lay down a low permeability filter cake which prevents the fluid in the cement from leaking-off to the formation. The most common fluid loss additives are members of the cellulose family of polymers. However, cellulose polymers retard the set time and increase the viscosity of cement. This retardation of setting can be excessive at low temperatures, and thus, expensive rig time is involved waiting on cement to set. Since these polymers also increase the viscosity, surface mixing can be difficult. To help with surface mixing, extra water is added to lower the surface viscosity. However, when the slurry is pumped down hole, the slurry sees increased temperature, and thermal thinning of the cellulose polymer occurs. This causes the slurry to become too thin, allowing solids to settle out.
Currently used fluid loss additives are satisfactory up to a point, but there is no universal additive that is effective at all temperatures in both freshwater and brine cement slurries.
2. The Prior Art
Polyvinyl alcohol, or more accurately polyvinyl alcohol-polyvinyl acetate polymer, has been used in fluid loss control for some period of time. A very detailed description of polyvinyl alcohol chemistry is found in U.S. Pat. No. 4,569,395 to Carpenter.
U.S. Pat. No. 4,011,909 to Adams et al discloses a well cement slurry including borax and polyvinyl alcohol, although that reference is primarily directed to improving flow properties of the cement.
U.S. Pat. No. 2,576,955 to Ludwig discloses a low-water-loss cement slurry containing polyvinyl alcohol, boric acid, and tributyl phosphate. This reference considers the boric acid to function as a retarder.
None of the above-noted references disclose use of calcium sulfate as part of a fluid loss additive in a cement slurry, and applicants have found that a specific additive package, including polyvinyl alcohol, calcium sulfate, boric acid, and optionally a defoamer, provides good fluid loss control at a variety of conditions and in the presence of both freshwater slurries and brine slurries.